FIG. 11 is a diagram showing an example of an existing data supply system preferably applied to a widely distributed system. The data supply system (CDN: Contents Delivery Network) of FIG. 11 has a hierarchical logical tree structure corresponding to the number of users or a geographical range covered by the system, and performs appropriate content supply in response to each audience request from users. Accordingly, it is possible to implement high speed responsibility, efficient network usage, and the like.
The above supply means a process of transferring a content to another apparatus, and making the transferred content into a storage device of the relevant apparatus. In the following descriptions, the term “supply” is used to have such a meaning.
The data supply system of FIG. 11 has a three-tier (hierarchical) structure constituting of DCs (Data Centers) from a DC arranged in a metropolitan area to DCs arranged in middle-ranking provincial cities. However, the manner of forming a hierarchy and the number of tiers are not limited to those in the example.
Generally, based on the number of users covered by the relevant system, the size of each DC has a general relationship “Tier 1>Tier 2>Tier 3”, that is, the higher the tier, the larger the size of each DC belonging to the relevant tier. Additionally, the capacity for contents storable in each DC has a similar relationship.
In such a case, each DC in Tier 3 stores part of contents stored in a DC belonging to Tier 1 or Tier 2.
In order to optimize the data supply system with respect to total responsibility, network usage efficiency, or the like, it is important to appropriately determine which DC each content will be supplied to, within a specific cost. That has been a problem to be solved.
For example, if users, who are connected to a middle-ranking provincial city DC1 and a high-ranking provincial city DC1, frequently access content A which is stored only in a metropolitan DC, then content A is provided from the metropolitan DC to the high-ranking provincial city DC1 in response to each access from the users who are connected to the high-ranking provincial city DC1.
Additionally, in response to each access from the users who are connected to the middle-ranking provincial city DC1, content A is provided from the metropolitan DC to the relevant high-ranking provincial city DC1, and is further provided from the high-ranking provincial city DC1 to the relevant middle-ranking provincial city DC1.
On the other hand, if content A has been supplied to the high-ranking provincial city DC1 which now stores content A, then content A can be directly supplied to a user, who is connected to the high-ranking provincial city DC1 and accesses content A, without receiving the relevant content from another DC. Additionally, for the access from a user connected to the middle-ranking provincial city DC1, content A is provided from the high-ranking provincial city DC1 to the middle-ranking provincial city DC1.
That is, in comparison with a case in that only the metropolitan DC stores content A, a higher level of network usage efficiency can be obtained. However, since there is the above-described general relationship such that the lower the tier, the smaller the size of each DC belonging to the relevant tier, it is not preferable to supply all contents to all DCs.
FIG. 12 is a diagram showing a known method for clearly designating DCs to which each content should be supplied.
In a simple approach for the content supply designation (see FIG. 12), DCs to which each content should be supplied may be clearly designated. However, this method cannot solve the following problems.    (1) Since it is necessary to determine for each DC whether or not content supply is performed, a considerable amount of cost occurs for the relevant computation.    (2) Since whether or not content supply is performed is clearly designated for each DC, an additional process to handle the increase or decrease in DCs must be implemented.    (3) Since the content supply information is statically designated, it is difficult to perform appropriate content supply in response to each demand for contents, for example, to supply a content to DCs belonging to a region which frequently accesses the content.    (4) Since information such as content viewing characteristics is not reflected in the relevant content supply designation method, the supply destination must be examined or controlled for each content. In addition, reexamination may sometimes be necessary.
Systems for solving the above problems have been developed, and the following Patent Documents disclose such a system.
For example, Patent Document 1 discloses a system configured by adding a distribution destination information DB to an existing content distribution system. In the configured system, relevant contents are distributed to terminals registered in the distribution destination information DB, based on a prepared distribution schedule and distribution conditions.
As another example, Patent Document 2 discloses a technique for a data supply system, in which dynamic and optimum content resupply to an edge server, from which a user terminal device directly reads a content, is performed based on user's viewing records, thereby maintaining desired service quality and implementing efficient system operation.